If you are dealing with someone with children, do you think it is ever ok for YOU to discipline their child if they are defiant?

This is a question I wanted to ask for a long time. For the past few years I’ve always dealt with men who had kids, most of the time daughters. I personally don’t think that it is ok to discipline someone else’s child unless you have the valid permission. Do you agree with me? When in a relationship do you feel as though it is appropriate to discipline your significant other’s child when they are defiant?